Individual dwellings or residences produce domestic liquid effluent, also known as domestic wastewater. Many residences may form a community whose liquid effluents are treated by a centralized community sanitation facility. The residences of such communities are thus integrated into the centralized treatment network. However, in many cases, individual residences or groups of residences are not part of a centralized effluent treatment network, because existing centralized facilities are at or near capacity and/or the costs of incorporating the individual or group residences are prohibitive.
Until recently, individual installations for treating domestic wastewater coming from a residence were considered as a temporary solution until the residence could be included in a community treatment network. The majority of individual installations have included a septic tank, followed by a leachfield realised by embedding perforated conduits in gravel to allow the septic tank effluent to be distributed and infiltrated into the native soil. This type of installation requires soil properties that permit infiltration, a large infiltration surface and a large vertical separation between the leachfield and the ground water tables. Still, progressive accumulation of pollutants in the soil leads to failures of such installations, particularly in the long term.
Primary, Secondary and Tertiary Systems
Individual residences therefore may require a local sanitation facility for treating their liquid effluents. Local sanitation facilities normally include primary and secondary systems, and in some applications they may include a tertiary treatment system.
“Primary systems” usually include a septic tank where sedimentation and flotation are used to separate the coarse and floating solids from the source wastewater. Primary systems may also include a biological decomposition of organic material in anaerobic conditions. The treated liquid exiting the primary system often has suspended solids (SS) between 50 and 80 mg/L and a biological oxygen demand (BOD5, the amount of dissolved oxygen consumed in five days by biological processes breaking down organic matter) between about 140 and 200 mg/L; a concentration of fecal coliforms of about 1,000,000 CFU/100 ml; phosphate concentrations of 5 to 15 mg/L; and total nitrogen concentrations of about 40 to 100 mg/L.
“Secondary systems” receive the primary treated effluent and enable an additional reduction of suspended solids and biological oxygen demand. The secondary system ensures a degradation of the organic matter by an aerobic biological process combined with or followed by a step of physical separation by sedimentation or filtration of the biological residues that are produced. For domestic wastewater treatment facilities, this secondary step can be achieved by a number of types of set-ups, the most frequently found of which are vertical sand filters, aerobic treatment units with or without fixed film media, biofiltration systems with organic or synthetic media, and constructed wetlands. The secondary treated liquid effluents emitted from such secondary systems usually have SS between 15 and 30 mg/L; BOD5 between 15 and 30 mg/L; concentrations of fecal pathogens between 25,000 and 200,000 CFU/100 ml; and levels of nitrogen and phosphates that have not been significantly reduced.
“Tertiary systems” further treat the liquid effluent emitted from the secondary system. Tertiary systems may be designed to further reduce the SS and BOD5 of the liquid effluent, to disinfect the liquid and/or to reduce the available nutrients in the liquid. A variety of physical, biological and chemical techniques may be involved depending on the parameters to control and treat. The tertiary treated effluents usually have a SS and BOD5 below 10 mg/L, a coliform level below 200 CFU/100 mL, a total phosphate concentration below 1 mg/L and a total nitrogen reduction of 50%.
In the field of “onsite” wastewater treatment, the wastewaters produced by individual residences are characterized by large fluctuations in flow rates and loading during single days and are a function of the habits of the residents. Variations are also observed on weekly and monthly bases according to the permanent or seasonal occupation of the residence, weekend activities, vacation periods, etc. Thus, onsite treatment of residential or single-dwelling domestic wastewaters is quite different from networked community domestic wastewater treatment, in that the numerous members of the latter produce a buffering effect on the individual variations of each residence. Also, community facilities do not experience prolonged periods of zero input.
Pathogenic Organisms from Domestic Wastewater
Every year, people become ill from the consumption of or exposure to contaminated water. Many outbreaks of waterborne illness have been associated with the consumption of untreated or inadequately treated ground water.
The majority of the outbreaks for which an etiologic agent has been identified are caused by micro-organisms, including bacteria, viruses and parasites. Typically, about 10% or less of the outbreaks are caused by chemicals. Approximately half of the outbreaks generally have no identified etiologic agent, and are listed as acute gastrointestinal illness. It is likely that the majority of these are also caused by pathogenic micro-organisms.
Since the early seventies, most of the installations installed to treat wastewater generated by individual homes were based on onsite wastewater disposal systems. These installations include a septic tank connected to a secondary system such as soil leach field, biofilter, aerobic treatment unit and/or constructed wetland. Little treatment of microbial pathogens occurs in the septic tank. Biological stabilization and pathogen removal mostly take place in the secondary system. Some of these systems are effective in reducing some microbes and pathogens.
The four types of pathogenic micro-organisms potentially present in human excreta are viruses, bacteria, protozoa and helminth eggs.
Viruses are very small, between about 0.02 and 0.10 μm, and are intracellular parasites made of nucleic acids (RNA or DNA) enclosed in a protein capsid. Inside a host, viruses divert most of the hosts' cellular machinery into viral replication until cell death. Outside of a host, viruses behave as abiotic colloidal particles. Because of their replication mechanism, viruses are very host specific. The types of viruses found in septic tank effluent are not all pathogenic to human cells. Most of them are called enteric bacteriophages because they need some specific gastrointestinal bacteria to multiply. However, some species are specific to human cells. They can cause a wide variety of diseases ranging from gastroenteritis to infectious hepatitis.
Bacteria are prokaryotic cellular organisms from about 0.2 to about 6 μm in size. The majority of bacteria in septic tank effluent are not true pathogens. Most are the normal flora which resides in the gut. However, some enteric bacterial pathogens can cause diseases ranging from gastroenteritis to ulcers to typhoid fever.
Protozoa are unicellular eukaryotic organisms from about 1 to 15 μm in size. They are generally shed from the gut in an environmentally stable cyst form. Diseases caused by enteric protozoa include gastroenteritis and dysentery.
Helminthes are intestinal worms. They are multicellular eukaryotic parasites. Helminth ova, which are about 30-100 μm in size, may be shed in feces.
As mentioned above, initial pathogen treatment occurs in the septic tank and includes removal by the settling of feces. It may be that a greater percentage of protozoan cysts and helminthes ova may be removed than bacteria or viruses during this process due to their much larger size. Removal efficiencies for all four pathogen types range from 0 to 2 log10 in the septic tank. The number of pathogens in septic tank effluent may reach 5×105 to 2×106/100 mL. The addition of a secondary treatment after the septic tank increases the pathogens removal, but the four types of pathogens are potentially present in the secondary effluent. Even with the use of more efficient secondary systems, such as peat filter, remaining pathogens have been observed in the treated effluent. In that case, most of them are viruses as well as motile and smaller bacteria able to pass through the effective pore size of the filter.
Infiltration of primary or secondary treated liquids within the soil involves disadvantages, particularly in certain soil conditions and characteristics. In fact, ideal sites for wastewater infiltration are less and less numerous. Furthermore, the erection of new residences in areas with certain environmental constraints such as low soil permeability, proximity to ground water table, etc., makes many known treatment installations insufficient.
Infiltrating secondary effluent into the soil, a known practice for a domestic onsite installation, has the potential to degrade groundwater quality depending on the limitations of the soil itself and the particular infiltration method. Even if some degree of disinfection may occur as wastewater percolates through soil presenting good infiltration capacity, known infiltration systems present difficulties especially in limitative soil and domestic wastewater applications.
In many cases, tertiary treatment is indeed desirable for further disinfection for onsite domestic wastewater treatment, mainly when the soil conditions do not allow infiltration for final disposal.
Types of Tertiary Systems
There are several different types of tertiary systems known in the field. For instance, there have been disinfection systems using filtration membranes, chlorination, UV light treatment, or ozone disinfection. These systems have several drawbacks such as toxicity, fouling concerns, management intensiveness, inefficiency, and being not cost effective for individual or small group facilities.
Passive systems have also been used for tertiary treatment of liquid effluents. For instance, biological systems such as biofilters or constructed filtering wetlands with long liquid retention times have been used to treat liquid coming from a secondary treatment system.
Passive Treatment Systems
Some onsite residential passive filter systems have been used for disinfecting effluent. The operation of such filters consists of a solid-liquid-gas triphasic system.
Principles of Passive Triphasic Systems
Equation 1 shows the relation existing between the three phases (solid, liquid, gas) in terms of hold-up corresponding to a fraction of the total volume of the reactor occupied by each of these phases.1=εS+εL+εg  (1)
The solid hold-up, or εS, can be subdivided in three components.εs=εsm+εsb+εsp  (2)where εsm corresponds to the fraction of solid volume occupied by the filtering material, εsb corresponds to the fraction of solid volume occupied by the biomass and εsp corresponds to the fraction of volume occupied by the particulate materials retained in the trickling bed.
The liquid hold-up, or εL, can be subdivided in two components:εL=εLs+εLd  (3)where εLd corresponds to the fraction of liquid volume occupied by the liquid in movement or flowing and εLs corresponds to the fraction of liquid volume occupied by the static liquid held up in the trickling bed.
In the same way, gaseous hold-up, or εg can be subdivided in two components, that is a static component (εgs) and a dynamic component (εgd):εg=εgs+εgd  (4)
Vertical Sand Filters
Vertical sand filters were originally developed for filtration of potable water after a first treatment of coagulation/flocculation to retain the fine particles from that chemical treatment. Such vertical sand filters are operated in saturated mode with a frequent counter-current washing. For wastewater treatment, most vertical sand filters are operated in a percolating non-saturated mode and have been usually used to treat liquids emitted from a primary treatment system. In percolating mode, vertical sand filters must have a surface area and height sufficient to ensure the necessary retention time of the liquid, to promote the various phenomena implicated in wastewater disinfection in aerobic conditions. However, considering the hydrodynamic conditions in vertical flow filters, which may involve hydraulic breakage and/or upward capillary dispersion, vertical filters for tertiary treatment of domestic liquid effluents used at a hydraulic loading rate around 50 L/m2/day, would need to be 600 to 900 mm high to ensure a sufficient non-saturated zone. Considering the surface area required, the efficiency of the vertical sand filters is also dependant on a uniform distribution of the wastewater over the top surface, which would require a controlled distribution system under low pressure. Vertical sand filters have several drawbacks when it comes to tertiary treatment.
Vertical-Horizontal Sand Filter
Referring to FIG. 1 (PRIOR ART), a vertical-horizontal sand filter A has also been used as a treatment system, in particular for completing the disinfection of an effluent coming from a constructed wetland, which offers a long retention time. This effluent contains less than 2 mg/L in SS and BOD5, and has an average fecal coliform concentration of 4 CFU/100 ml, which already conforms to tertiary treatment standards at the entrance to the vertical-horizontal sand filter A.
This vertical-horizontal sand filter A has an inlet means B that feeds the wastewater vertically onto the horizontal sand-packed section C, with a length of about 3 m, and the wastewater flows horizontally downstream to the outlet D. This vertical-horizontal sand filter A has a short horizontal distance and a low retention time offering limited disinfection capacity. Also, the gravitational inlet means B enables limited distribution of the effluent and is embedded within the sand packing, which would increase the chance of undesirable flow channels being formed. In such liquid-saturated flow channels, a dynamic liquid hold-up (εLd) would dominate limiting the gas hold-up (εg). Utilizing this sand filter A with a more polluted secondary effluent, having for instance a suspended solid matter and BOD5 of about 30 mg/L, would lead to the development in the liquid-saturated zones of an anaerobic clogging biofilm (εsb) due to the reduction in gas hold-up (εg). With time, progressive clogging of the entrance would occur, from the accumulation of biomass and suspended solids, leading to the failure of the filter A.
The sand-packed section C is covered with top soil E, which leads to disadvantages in maintaining and ensuring that sufficient aerobic biological disinfection occurs in the sand-packed section C. An impermeable geo-membrane F may be provided below and above the sand-packed section C. However, this vertical-horizontal sand filter A may become smothered and soaked with precipitation from above and/or blocked up from the soil's freezing or becoming covered with snow or ice. Such effects adversely influence the aerobic disinfection by not allowing sufficient aeration in the surrounding soil E and/or the sand-packed section C.
The tertiary wastewater treatment systems that have been used in the field have disadvantages such as lack of longevity, incomplete or unreliable treatment, limited adaptability to different soil conditions and characteristics, particularly in sensitive environments such as lakeside properties, and limited adaptability to fluctuations in liquid effluent input.
The technologies in the field have several disadvantages in treating secondary liquid effluent. There is indeed a need in the field of tertiary treatment systems in domestic applications for an improved technology that can overcome at least some of the disadvantages of what is known in the field.